In devices of this type, it is possible to further process thin-walled wastes of thermoplastic plastics, as for example foil residues, shopping bags or wastes which are created during blowing of plastics, to a plastic granulated material which can be used over again. At suitably high drive powers for the crushing tool, it is possible to obtain the temperature which is required for the plastification exclusively by producing frictional heat, which is created by the friction that exists between the tool and the wastes. This method for plastification of plastic wastes is known (German Pat. No. 1,118,959, British Pat. No. 970,822).
Also a device of the above-mentioned type is known (German AUS No. 1,679,834, corresponds to U.S. Pat. No. 3,510,067), which has a cylindrical container, above the bottom of which, at a small distance from same, there is rotatably arranged a beating tool which has two beaters offset relative to another by 180.degree. and the beaters are equipped with knives. Crushing members are arranged on the container wall, which crushing members lie approximately in the same plant as the beaters. This known device requires a relatively long time for processing a charge of plastic wastes to granulated material because the circulation of the wastes in the container is relatively slow. The required driving power is great in relationship to the amount of heat required for plastification because the frictional heat is created for a good part during a pressing of the plastic between the beaters and the crushing tools and for an important part is discharged outwardly through the crushing members, which for the purpose of adjustability extend through the container wall.
The basic purpose of the invention is to further develop a device of the above-mentioned type so that at a given processing output of the device, the power need for the drive of the crushing tools is less than in the conventional device.
This purpose is attained according to the invention by providing a crushing tool having at least two rotatable shafts, which are both equipped with beaters, wherein the peripheral circles described by the outer ends of the beaters, as viewed from the top, lie closely together or are overlapped and, when overlapped, the beaters of adjacent shafts are vertically offset relative to one another.
In such a constructed device, the crushing operation no longer takes place near the container wall but where the beaters approach one another. If the beaters rotate in the same directions, as this is preferably the case, there is created a movement against one another like a cutting movement of a scissor. Thus, a very good crushing action is achieved. The conversion of mechanical energy into frictional heat does not take place near the container wall as in the conventional device, so that also the heat discharge to the outside is less. The required driving power for the crushing tools is less than in the conventional device because the cutting operation is better and the heat discharge to the outside is reduced. In the cutting zone, namely in the area in which the peripheral circles contact or intersect, there is also created a type of a suction action, which is favorable for the circulation of the wastes in the container, through which are achieved shorter processing times for a given charge. By using more than one beater knife, the structural height is also reduced.
Each shaft is driven by a separate motor which runs independently from a drive motor for the other shaft. Preferably, the beaters are mounted directly on the motor shaft. However, it is also possible for the shafts to be coupled to one another, preferably so that they have the same rotational speeds and the beaters simultaneously closely approach one another or are overlapped at an overlapping zone for the peripheral circles. Thus, a maximum of cutting movements is achieved. However, the practical test has shown that a mechanical coupling of the shafts is not needed and this has the advantage of a substantial simplification and reduction in the price of the construction.
Two beaters which are offset against one another at 180.degree. can be arranged on each shaft. However, it is also possible to have more than two beaters per shaft. The beaters may have cutting edges of a conventional manner, which are preferably provided on exchangeable and turnable knives, which are sharpened on all sides.
It has been proven to be advantageous for the beaters to have cutting edges at their outer ends, preferably straight cutting edges, which extend substantially perpendicular to the radial direction. The great effectiveness of these outer cutting edges is mainly due to the fact that the plastic wastes, during the movement of the two beaters against one another, are gripped between said beaters.
In a preferred embodiment of the invention, the crushing tool has two shafts and the container wall has an oval cross-sectional shape, which is composed of two semicircles which are spaced a small distance from the peripheral circles of the beaters and of straight portions connecting the semicircular portions. The container can also be composed of two U-shaped sheet metal parts joined together. This design can be constructed easily and inexpensively. In the case of a greater need of processing output, it is possible, however, to provide also more than two shafts with beaters.